Systems and methods for vessel position reporting and monitoring

ABSTRACT

Systems and methods for vessel position reporting and monitoring. Methods and systems for augmenting e-Navigation messages to provide ancillary information, such as a history of previous vessel positions. A compact representation is provided, in which transmitters may select among a plurality of possible position layouts to provide a compact representation. Transmitted messages are received by a satellite or other surveillance platform employing a compatible radio frequency receiver to collect message signals over a large area or great distance.

FIELD

The described embodiments relate to tracking of vessels and otherobjects and, more particularly, to methods and systems for providingenhanced position reports in tracking messages.

BACKGROUND

Vessels and other objects that can change position over time may betracked using various systems and approaches. For example, E-Navigationis a concept developed by the International Maritime Organization (IMO)to improve safety and security of commercial shipping operations byfacilitating better communication and organization of data between shipsand shore-based facilities.

E-Navigation has been defined as the “harmonized collection,integration, exchange, presentation and analysis of marine informationonboard and ashore by electronic means to enhance berth to berthnavigation and related services for safety and security at sea andprotection of the marine environment.”

As such, e-Navigation incorporates and expands on a number oftechnologies, including the Automatic Identification System (AIS),Electronic Chart Display and Information System (ECDIS), IntegratedBridge Systems/Integrated Navigation Systems (IBS/INS), Automatic RadarPlotting Aids (ARPA), Long Range Identification and Tracking (LRIT)systems, Vessel Traffic Services (VTS) and the Global Maritime DistressSafety System (GMDSS).

SUMMARY

In a first broad aspect, there is provided a method of transmitting amessage that identifies a geographical position, the method comprising:encoding a track history to generate an encoded track history;generating the message comprising the encoded track history; andtransmitting the message.

In some cases, the method may further comprise updating the trackhistory based on the geographical position or course over ground, orboth.

In some cases, the method may further comprise setting a message typeidentifier, wherein the message further comprises the message typeidentifier.

In some cases, the method may further comprise retrieving a transmitteridentifier, wherein the message further comprises the transmitteridentifier.

In some cases, the track history comprises one or more track historypoints based on prior geographical positions. In some cases, each of theone or more track history points are separated by a preselected timeinterval.

In some cases, each of the track history points is determined withreference to a predetermined point layout that has a plurality ofpredefined point positions. In some cases, each of the track historypoints is selected by approximating the position associated with aprevious message to one of the predefined point layout positions.

In some cases, the method may further comprise selecting thepredetermined point layout from a plurality of possible point layouts.In some cases, the predetermined point layout is selected by computingminimum mean square error of the one or more track history pointsrelative to the predefined point positions in each of the plurality ofpossible point layouts. In some cases, the predetermined point layout isselected by computing maximum error of the one or more track historypoints relative to the predefined point positions in each of theplurality of possible point layouts.

In some cases, the method may further comprise selecting a firstdimensional component value array, wherein the first dimensionalcomponent value array defines the values of successive points in thepredetermined point layout.

In some cases, the first dimensional component value arrays are definedin a first look up table having a first index. In some cases, each trackhistory point in the track history is identified at least by arespective index value from the first index. In some cases, each of therespective index values is Huffman encoded in the track history.

In some cases, the method may further comprise selecting a seconddimensional component value array, wherein the second dimensionalcomponent value array defines the values of successive points in thepredetermined point layout.

In some cases, the second dimensional component value arrays are definedin a second look up table having a second index. In some cases, thefirst and second indices are selected in a point layout look-up table.

In some cases, at least one of the track history points comprises analert flag tag.

In some cases, the encoding comprises determining the track history is apartial track history that includes less than a preset number of trackhistory points.

In some cases, the message comprises a binary payload and the encodedtrack history is comprised in the binary payload.

In some cases, the binary payload comprises at least one of an alertidentifier, a course over ground indication, a latitude and longitudeindication and a cyclic redundancy check code.

In some cases, the method may further comprise error protection encodingthe binary payload.

In another broad aspect, there is provided an apparatus for transmittinga message that identifies a geographical position, the apparatuscomprising: a memory that stores a track history; a processor configuredto: encode the track history to generate an encoded track history;generate the message comprising the encoded track history; and atransmitter that transmits the message.

In another broad aspect, there is provided a system for transmitting andreceiving track histories, the system comprising: a transmitting device,the transmitting device comprising: a memory that stores a trackhistory; a processor configured to: encode the track history to generatean encoded track history; generate the message comprising the encodedtrack history; and a transmitter that transmits the message in a signal;and a receiving device, the receiving device comprising: a receiver, thereceiver configured to receive the signal and identify the message; aprocessor configured to decode the encoded track history and obtain thetrack history.

In another broad aspect, there is provided a non-transitory computerreadable medium storing computer executable instructions, theinstructions, when executed by a computer processor, for causing thecomputer processor to perform a method of transmitting a message thatidentifies a geographical position, the method comprising: encoding atrack history to generate an encoded track history; generating themessage comprising the encoded track history; and controlling atransmitter to transmit the message.

In another broad aspect, there is provided a method of processing amessage, the method comprising: receiving a first message, the firstmessage comprising a track history that has error-protection encodingapplied thereto; storing the first message when the error-protectionencoding cannot be successfully decoded; receiving a second message, thesecond message containing the track history with error-protectingencoding applied thereto; storing the second message when theerror-protection encoding cannot be successfully decoded; determiningthat the first message and the second message belong to a common burstsequence; processing the first message and the second message togenerate a recovered message; and decoding the error-protection encodingof the recovered message to obtain the track history.

In another broad aspect, there is provided a system for processing amessage, the system comprising: a memory; a processor configured to:receive a first message, the first message comprising a track historythat has error-protection encoding applied thereto; store the firstmessage when the error-protection encoding cannot be successfullydecoded; receive a second message, the second message containing thetrack history with error-protecting encoding applied thereto; store thesecond message when the error-protection encoding cannot be successfullydecoded; determine that the first message and the second message belongto a common burst sequence; process the first message and the secondmessage to generate a recovered message; and decode the error-protectionencoding of the recovered message to obtain the track history.

In another broad aspect, there is provided a method of processing amessage, the method comprising: receiving a first message, the firstmessage comprising a track history, wherein each of the track historypoints is determined with reference to a predetermined point layoutdefined in the first message, the predetermined point layout having aplurality of predefined point positions; and processing the firstmessage to obtain the track history.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the various embodiments described herein,and to show more clearly how they may be carried into effect, referencewill now be made, by way of example only, to the accompanying drawingsin which:

FIG. 1 is a simplified schematic diagram of an example for satelliteaspects of e-Navigation system;

FIG. 2 is a system block diagram of an example e-Navigation transmitterfor the system of FIG. 1;

FIG. 3 is a system block diagram of an example e-Navigation receiver forthe system of FIG. 1;

FIG. 4A is a diagram illustrating the data structure of an examplee-Navigation message signal;

FIG. 4B is a diagram illustrating the data structure of the payload ofthe example e-Navigation message signal of FIG. 4A;

FIG. 5 is a diagram of an example point layout;

FIG. 6 is a diagram of a track history generated using the example pointlayout of FIG. 5;

FIG. 7 is a functional flow diagram for an example method of generatingan e-Navigation message;

FIG. 8 is a functional flow diagram for an example method of receivingand processing an e-Navigation message with enhanced detectability; and

FIG. 9 is a diagram of another example point layout.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

It will be appreciated that for simplicity and clarity of illustration,where considered appropriate, reference numerals may be repeated amongthe figures to indicate corresponding or analogous elements or steps. Inaddition, numerous specific details are set forth in order to provide athorough understanding of the exemplary embodiments described herein.However, it will be understood by those of ordinary skill in the artthat the embodiments described herein may be practiced without thesespecific details. In other instances, well-known methods, procedures andcomponents have not been described in detail since these are known tothose skilled in the art. Furthermore, it should be noted that thisdescription is not intended to limit the scope of the embodimentsdescribed herein, but rather as merely describing one or more exemplaryimplementations.

Messaging between ships and other stations is a core requirement ofe-Navigation. In part due to the mandated use of AIS in commercialshipping, the developers of e-Navigation have selected AIS as themessaging platform for e-Navigation.

AIS is a Radio Frequency (RF)-based communications and collisionavoidance system for large SOLAS (Safety of Life at Sea) class vessels.

Since 2004, the IMO has required AIS transponders to be aboard allvessels that exceed 300 gross tons or passenger vessels of any size.Class A AIS transceivers are designed for these large ships.Additionally, AIS technology is increasingly being deployed in smallervessels and is also being installed in Aids-To-Navigation (AtoN) andSearch and Rescue (SAR) vessels and aircraft. Since the deployment ofAIS, the IMO has begun the process of defining e-Navigation.E-Navigation uses the AIS communication system to facilitate variouse-Navigation requirements, and specifically will require the heavy usageof Application Specific Messages (ASMs).

A Class A transceiver transmits with a power of up to 12.5 W, which issufficient for short-range (typically 20-30 nautical miles) ship-to-shipand ship-to-shore communications. The system uses narrowband (i.e. 25kHz Bandwidth) Very High Frequency (VHF) channels, at 161.975 MHz and162.025 MHz for example, and a communication method calledSelf-Organizing Time Division Multiple Access (SOTDMA). The ITU has alsoreserved additional channels for the use of AIS to support e-Navigation,and other organizing/access schemes may also be used in some cases.

Each AIS transceiver also includes a receiver component, enabling a shipto receive AIS signals emitted by ships around it. Each minute, each VHFchannel is divided into 2,250 time slots, each of which can accommodateone 26.67 ms AIS transmission (i.e. AIS signal). The time slots areaccurately synchronized to Coordinated Universal Time (UTC), typicallyusing a Global Navigation Satellite System (GNSS), such as the GlobalPositioning System (GPS). Each AIS unit reserves time slots for futureAIS transmissions from the ship. Other AIS units within range cantherefore maintain a map of reserved slots and avoid transmitting duringthese intervals. This self-organizing feature avoids signal collisionsover the short ranges involved in surface transmissions.

Class A transceivers are high powered and some vessels may lack on-boardpower sufficient to operate the transceiver for long journeys. Class Atransceivers may also be prohibitively expensive for smaller vessels.Accordingly, the AIS standard also defines a second class oftransceiver, called Class B, which is intended for smaller vessels. AClass B transceiver typically includes one VHF transmitter, two VHFCarrier Sense Time Division Multiple Access (CSTDMA) receivers, whichcan alternate as VHF Digital Selective Calling (DSC) receivers, and aGNSS receiver. Some Class B transceivers may use the SOTDMA accessscheme or above simpler carrier-sense (CS) access scheme to share thechannel with Class A transceivers. The Class B transceiver transmits astandard AIS data stream, as with Class A transceivers. However, incontrast to Class A transceivers, a Class B transceiver is limited to 2W of transmission power, combined with a lower antenna mast heighttypical of smaller vessels, the conventional range of a Class Btransceiver is lower than most Class A transceivers.

CSTDMA transceivers listen for other transmissions and attempt toidentify slots that are not in use by another AIS transmitter.Typically, Class B transmitters have a default transmit rate of onemessage per 30 seconds, although this can be varied.

Ships employing AIS broadcast key details every few seconds about theiridentification, current position, heading and speed, allowing the shipsto select a safe course even when conditions prevent visual contact. TheAIS system supports a number of different types of signal.

Conventionally, the principal AIS signal sent by a ship is a positionreport that provides information pertaining to the ship'sidentification, location, course, speed, and other details. Morerecently, ASMs allow for the definition of additional AIS messagesubtypes. Through the use of ASMs, AIS can be extended or leveraged totransmit a variety of other types of data, as described further herein.This extensibility is leveraged by e-Navigation to transmit other typesof data. For example, IMO defines ASM subtypes for transmitting weatherobservation reports from ships, maritime traffic signals and berthingdata, among others. ASMs can also be used to communicate free-formbinary payloads.

The IMO, and others, have recognized that the use of ASMs within AIS ande-Navigation will continue to increase causing an ever more congestedenvironment on the VDL (VHF Data Link) used for AIS and e-Navigation.This problem is particularly exacerbated in the case of satellitedetection of AIS signals.

Although AIS was developed as a self-organizing system within a localradius, AIS signals can also be detected using low earth orbit (LEO)satellites. LEO satellites have a large field of view (FOV), typicallymeasured in thousands of nautical miles, which means that the LEOsatellite may receive signals from large numbers of ships at once. Dueto the large FOV, the self-organizing features of AIS are not sufficientto prevent signal collisions in this scenario. As a result, LEOsatellite reception leads to a large number of AIS signals colliding oroverlapping with one another.

Moreover, many AIS signals may not be transmitted with sufficient powerto be received without error at an LEO satellite. AIS Class Atransmitters, such as those aboard large commercial vessels, have atransmit power level of 12.5 W, by default, which may not be sufficientto overcome noise or signal collisions in many cases. However, AIS ClassB transmitters, which can be found on small vessels, have lesssophisticated transmitters limited to a power level of just 2 W andwhich transmit infrequently, making their detection difficult in areaswith a high vessel density and significant AIS traffic. The largefootprint of a satellite receiver can further complicate detection.

Nevertheless, the population of small vessels is continuing to grow onthe VDL. Small vessels are of particular interest to securityorganizations as their ownership and purposes within national waters canbe difficult to trace given that they operate outside the IMO and theSOLAS conventions. It is desired to use AIS, and LEO satellitedetection, to track these vessels but their low power and infrequenttransmissions make it very difficult to detect these vessels,particularly in congested areas.

The described embodiments are generally directed to generating andtransmitting e-Navigation (e.g., AIS) signals with a compatible messageformat that encodes a position history, allowing for a vessel track tobe determined even if relatively few transmitted messages aresuccessfully received and decoded. By transmitting a recent trackhistory, even small vessels can be monitored even when a large number oftheir position reports are lost in a noisy environment, or otherwisefail to be received and processed by a monitoring system. However, thedescribed concepts are generally applicable to both AIS Class A andClass B transceivers.

In addition, other techniques can be used to significantly improve thelikelihood of reception even when transmitted at low power, such forwarderror correction coding and the like.

The described embodiments can be provided in such a way as to becompliant with existing AIS standards, and backward compatible withconventional AIS data communications transponders, such as used anddefined within the AIS standard, and in evolving e-Navigation standards.

It will be appreciated that although the example embodiments herein aredescribed with reference to AIS, the described concepts may also beapplicable to other e-Navigation standards. Moreover, certain aspects(e.g., modulation scheme) of AIS and e-Navigation may be modified in thefuture.

Similarly, the described concepts may also be applicable to otherposition tracking systems, including but not limited to motor vehicletracking over land, rail car tracking, aircraft tracking and others.

Referring now to FIG. 1, there is shown a simplified schematic diagramof an example e-Navigation system 100. System 100 has at least one LEOsatellite 150, which receives e-Navigation message signals transmittedby one or more ship-based transmitters 110, one or more small vesseltransmitters 115, and other transmitters, such as a ground-basedtransmitter 120. LEO satellite 150 can transmit data corresponding tothe received message signals to a ground station 180. In someembodiments, LEO satellite 150 may perform post-processing and recoveryof the received message signals on-board. In other embodiments, LEOsatellite 150 may re-transmit raw signal data to one or more groundstations 180 for post-processing and recovery.

In embodiments where e-Navigation employs AIS-based communications, agiven Class A ship transmitter 110 will typically transmit AIS signalsover a number of narrowband (i.e. 25 kHz) VHF channels. Examples of AISVHF channels include AIS1 at 161.975 MHz and AIS2 at 162.025 MHz. Totransmit an AIS signal, the transmitting unit employs a 9.6 kbpsGaussian minimum shift keying (GMSK) modulation.

Similarly, a Class B ship transmitter 115 will transmit AIS signalsusing CSTDMA or SOTDMA over VHF channels using an appropriate modulationand coding.

As noted above, other AIS channels also exist and may be used, and stillfurther channels—which may use varying access schemes, modulation typesand channel bandwidth—may be added globally or regionally from time totime, in order to facilitate e-Navigation or maritime communicationservices. The present embodiments may be used with any combination ofchannels.

The LEO satellite 150 is equipped with at least one VHF antenna andreceives the AIS signal transmitted by the transmitter 110 ortransmitter 115. The LEO satellite 150 travels at a high orbitalvelocity, such as 7,500 m/s for example, and consequently the AIS signalreceived by the LEO satellite 150 may undergo a Doppler shift of up to+/−5.0 kHz.

In the embodiments described herein, transmitters 110, 115 and 120 maygenerate and transmit AIS signals as described with reference to FIGS.5A and 5B.

Received AIS signals may be filtered and pre-processed using knowntechniques for receiving AIS messages. For example, the received AISsignal may be narrowband filtered to isolate the AIS channels ofinterest. The receiver may then attempt to identify a candidate messagein the signal data, for example by locating an AIS training sequence inthe signal data.

Once a candidate message is identified, GMSK demodulation may beperformed followed by Non-Return to Zero Inverted (NRZI) decoding, bitde-stuffing and bit reversal to recover the AIS message bit sequence.

The AIS message cyclic redundancy check (CRC) code may be verified atthis stage, to determine if any errors are present in the recovered AISmessage bit sequence. Conventionally in AIS message processing, if theCRC verification fails, the message data is discarded. However, in theembodiments presented here, processing can continue even in the presenceof one or more bit errors in the AIS candidate message. Accordingly, thetop level CRC verification may be omitted in some embodiments describedherein.

Once the AIS message bit sequence is recovered, the system proceeds topayload processing for each AIS message bit sequence, as described withreference to FIG. 8.

Referring now to FIG. 2, there is illustrated a system block diagram foran e-Navigation transmitter in some example embodiments. E-Navigationtransmitter 200 may be used in ship transmitter 110, small vesseltransmitter 115 and ground transmitter 120, for example.

Transmitter 200 includes a processor 220, memory 225 (which may includevolatile and non-volatile memory) and an RF transmitter stage 210. RFtransmitter stage 210 includes one or more amplifiers, filters andantennas, for example, used to transmit an AIS message signal. In someembodiments, RF transmitter stage 210 operates at low power levels(e.g., AIS Class B standard power level of 2 W, or less). In some otherembodiments, RF transmitter stage 210 may operate at AIS Class A powerlevels (e.g., 12.5 W).

Processor 220 may be a general purpose microprocessor ormicro-controller, field programmable gate array, application specificprocessor, or other computing device. Processor 220 can be configured toperform various tasks, such as monitoring position data from positionmodule 250, determining navigational status, generating maneuverindications, maintaining historical course information (e.g., stored inmemory 225). It will be appreciated that processor 220 can performvarious other tasks dependent on available or installed software. Basedon these tasks, processor 220 can generate message data for transmissionin an e-Navigation message. Message data need not be limited to AISposition reports, and may include any free-form data usable ine-Navigation.

Transmitter 200 further comprises a position module 250, a message datamodule 255, an error protection module 260 a message formatting module265 and a track history module 270. In some embodiments, variousmodules, such as error protection module 260 and message formattingmodule 265, or position module 250 and track history module 270, may becombined into a single module, or may be divided into varioussub-modules.

Position module 250 is a receiver for a global satellite navigationsystem (GNSS), such as the Global Positioning System (GPS), Galileo, orthe Global Navigation Satellite System (GLONASS). In some cases,position module 250 may instead be a cellular, FM-radio based,inertial-based or other type of position determination device. Positionmodule 250 is generally capable of determining a present location oftransmitter 200 and a current time, and supplying this data to processor220.

Message data module 255 receives message data from processor 220 andgenerates binary payload data, based on the received message data, forinclusion in an AIS signal transmission.

Error protection module 260 receives the binary payload data and encodesthe binary payload data using at least one error protection scheme asdescribed with reference to FIG. 7.

Message formatting module 265 receives the encoded binary payload dataand encapsulates the encoded binary payload data in a message wrapperfor transmission, as described with reference to FIG. 7. For example,the message wrapper may be a properly-formatted AIS ASM. In someembodiments, other parts of the message may be encoded in addition tothe binary payload portion. In some embodiments, all data in themessage, with the except of a synchronization ‘preamble’ or trainingsequence may be encoded.

Track history module 270 receives message data from position module 250or processor 220, and can generate a track history based on theavailable message data, as described with reference to FIG. 7.

In some embodiments, transmitter 200 may provide an enhanced level ofmessage detection compared to standard AIS Class B transmitters (at thesame RF power level) in the presence of: other AIS transmitters, or inareas with a high density of like messaging transmitters.

Even where some messages are not detected successfully, the inclusion oftrack history in e-Navigation messages facilitates the continuousmonitoring of vessels.

Transmitter 200 can be compatible with existing AIS modulation,channelization, message standards and signaling protocols, and relatedlyno new modulation scheme or digital baseband circuitry is required forthe transceiver.

Transmitter 200 has low processing load requirements to encode messages.Thus microcontrollers can be used, avoiding the need for advancedprocessors and/or floating point calculations.

Transmitter 200 can be configured for different types of behaviors atthe factory or in the field via a firmware upgrade. Accordingly,transmitter 200 can be configured to operate in a ‘closed’ system, inwhich all e-Navigation messages it sends are proprietary. Alternatively,transmitter 200 can operate in a hybrid mode, by providing some or allenhanced features to a compatible network (ground, satellite, etc.)infrastructure, while still occasionally transmitting standard AISposition reports as required. Such behavior can be dedicatedrespectively to different channels (e.g., AIS ASM1, ASM2, etc.), or thehybrid mode can be employed on a single or pair of channels (such asstandard AIS frequencies).

Moreover, the behavior of transmitter 200 can optionally be modifiedover time via over-the-air programming (e.g., using e-Navigationmessages or AIS ASMs for this purpose), or over another wired orwireless link (e.g., Wi-Fi or USB).

Referring now to FIG. 3, there is illustrated a system block diagram foran e-Navigation receiver in some example embodiments. E-Navigationreceiver 300 may be used in LEO satellite 150 or ground station 180, forexample.

Receiver 300 includes a processor 330, memory 340 (which may includevolatile and non-volatile memory), one or more antennas 310, and an RFreceiver stage 320.

Receiver 300 further includes a message processing module 360, a payloadprocessing module 370 and a recovery module 380. In some embodiments,message processing module 360 and payload processing module 370 may becombined into a single module, or may be divided into varioussub-modules.

Processor 330 may be a general purpose microprocessor, fieldprogrammable gate array, application specific processor, or othercomputing device. Processor 330 can be configured to control theoperation of the various modules, and to receive and further processmessage data recovered from e-Navigation signals.

RF receiver stage 320 includes one or more filters and amplifiers topre-process signal data received by antenna 310. Pre-processed signaldata is provided to message processing module 360, which identifiescandidate message signals and performs GMSK demodulation followed byNon-Return to Zero Inverted (NRZI) decoding, de-stuffing and bitreversal to recover input signal data, for example an AIS message bitsequence.

Payload processing module 370 receives the input signal data and may, insome cases, extract the binary payload encapsulated within a messagewrapper. Payload processing module 370 can then decode any errorprotection encoding schemes of the binary payload to recover messagedata.

Messages that are not successfully decoded may be passed to recoverymodule 380, which may store them in memory 340 for later processing. Ifmultiple messages are received within a Burst Sequence, as describedherein, recovery module 380 may retrieve a plurality of messages for theburst sequence, and perform recovery processing to identify a messagethat can be successfully decoded. For example, a “voting” procedure canbe used, wherein bits that occur most frequently are selected. Repeatedmessages may also be summed, or a weighted “voting” procedure used foreach bit, to generate a composite message that may have fewer biterrors, and which can be successfully decoded.

Referring now to FIG. 4A, there is shown a diagram illustrating the datastructure of an example e-Navigation message signal 400. The AIS messagesignal 400 generally comprises a binary payload portion 410 encapsulatedwithin a message wrapper. The message wrapper is illustrated as having afirst wrapper portion 420A that precedes the binary, encoded payloadportion 410 in the message bit sequence, and a second wrapper portion420B that follows the binary payload portion 410 in the message bitsequence. For ease of understanding, the first wrapper portion 420A andsecond wrapper portion 420B are collectively referred to herein as themessage wrapper 420.

The message length of standard AIS messages is subject to someconstraints based on bit rate (i.e., 9.6 kbps) and slot length (i.e.,26.67 ms), however longer messages may be still be transmitted by usingtwo or more slots in the AIS system. In the example embodiments set outherein, the e-Navigation message is limited to 512 bits in length, whichcorresponds to two slots. However, in some cases, an e-Navigationmessage may occupy fewer or more than two slots. For example, byreducing the number of history points or eliminating them, a message mayoccupy only one slot. In addition, an e-Navigation message may occupyfractional time slots, for example one and a half time slots, and stillbe a valid AIS message.

Message wrapper 420 contains fields required to form a valid,standards-compliant AIS message; an example of a Type 8 AIS message isshown in Table 1 below. For example, an initial offset or ramp up field(8 bits), an AIS training sequence (24 bits), and a start flag (8 bits).A message identifier (6), repeat indicator (2), Maritime Mobile ServiceIdentity (MMSI) (30 bits), spare field (2 bits) and applicationidentifier (16 bits) may also be included. Finally, a frame checksequence (e.g., CRC) field, a buffer field or bit stuffing and a stopflag may be included to complete the AIS message.

TABLE 1 Field Number of bits Initial Transmit offset bits 8Preamble/training sequence 24  Start Flag 8 Message ID 6 RepeatIndicator 2 MMSI 30  Spare 2 Application ID 16  Encoded Payload 304* *152 decoded bits when using FEC at rate ½ Full Message CRC 16  StopFlag 8 End Buffer (bit stuffing) 88  (or less for bit stuffing) Total512 

Depending on the type of message, a different wrapper may be used. Forexample, an AIS Type 25 or Type 26 may have a modified wrapper, to suitthe different message parameters. The described approach can be appliedto many different message types, including but not limited to messagetypes defined in the future as new protocols are defined.

The AIS training sequence field is included to allow conventional AISreceivers to perform carrier recovery. The training sequence field, thestart flag field and the stop flag field are predetermined AIS codesequences that can be used to identify candidate message signals in adata stream.

The message identifier is a standard AIS message identifier, such as amessage identifier signifying a Type 8 message, which is defined in theAIS standard for miscellaneous use.

A Repeat Indicator can be used to indicate the number of times anidentical message has been transmitted.

A Spare field can be defined for future use, for example.

An Application ID, which is similar to a message identifier, can be usedto define message sub-types. For example, it may define sub-types of aType 8 message. In general, a unique Application ID can be assigned toall new and predefined message sub-types for a particular messageidentifier (e.g., Type 8 (binary) messages). For example, thee-Navigation messages as described herein may have a unique ApplicationID. If a new message format is defined, then a different Application IDmay be used.

The CRC check sequence field is used for error detection of the fullmessage (as opposed to the binary payload alone), and can be used todetermine whether bit errors are present in the overall AIS messagefollowing transmission and reception. The end buffer field may change inlength and is used to ensure that the overall AIS message signal lengthremains constant regardless of the number of bits added due tobitstuffing

In general, the binary payload 410 may contain predetermined free-formdata of any type. In the example embodiments, the binary payload 410 canbe sub-divided into a plurality of pre-defined fields as describedherein.

FIG. 4B is a diagram illustrating the data structure of the binarypayload 410, without forward error correction coding. The binary payload410 contains:

-   -   MMSI    -   Message sub-type identifier    -   Longitude    -   Latitude    -   Course over Ground (COG)    -   Burst Sequence Flag    -   Track History    -   Spare bits (if necessary)    -   Alert Flags (when possible)    -   Payload-level CRC for the unencoded/decoded binary payload

In the example embodiments, the field order does not change and adheresto the pattern above. In alternative embodiments, other orders orvariable orders may be used. In the example embodiments, most fields inthe unencoded payload are variable in length, with the exception ofMMSI, which is 30 bits in length. Other embodiments may use a shorter orlonger unique identifier instead of the MMSI commonly used ine-Navigation. However, in some alternative embodiments, one or moreother fields may be fixed in length. Fields that are similar to standardAIS position reports may have differing levels of numerical precisionand or encoding method, such as the longitutude and latitude fieldproposed in the embodiments described herein. In the describedembodiments, the resolution is decreased from AIS position report levelsto reduce data volume, which is acceptable for positional trackingpurposes, as opposed to collision avoidance. Adjustments in theprecision and types of fields presented can be made to meet the needs ofthe intended application.

In some embodiments, one or more of these fields may be omitted, whilein other embodiments, additional fields may be added. It can be seenthat the binary payload 410 may omit certain fields found in an AISposition report, such as radio status. However, certain fields may berepeated or redundant (e.g., MMSI) to those found in the wrapper, andnew payload fields may be added. A payload CRC field may also beprovided, that provides a check on the unencoded or decoded data of thebinary payload 410.

The payload message format is determined by the message sub-typeidentifier in the payload. The message sub-type identifier may bevariable in length and in the example embodiments varies between aminimum of 9 bits and a maximum of 14 bits. In particular, the messagesub-type identifier may have 9 bits when a complete track report of ninetrack history points is present (although in some embodiments fewer ormore than nine track history points may be present). Some otheroperating modes may also allow for a 9-bit message sub-type identifier.When an incomplete or partial track history is present, the messagesub-type identifier may be up to 14 bits in length.

In some example embodiments, the first bit of the message sub-typeidentifier defines whether or not the message is a “regular” messagethat contains a complete track history, with a predefined field order.For example, the first bit (e.g., most significant bit) may be 1 forcomplete track history, and 0 otherwise.

If the message is a regular message with a complete track history, theremaining eight bits of the message sub-type identifier may be a pointlayout selection index to select—from a predefined list—the appropriatepredetermined point layout or point layouts for representing the trackhistory contained in the message, as described herein. These eight bitscan represent up to 256 different predetermined point layoutpossibilities that can be placed in the predefined list with a varietyof different precisions. In practice, fewer than all 256 possible pointlayouts need be defined. For example, combinations of 5-bit and 9-bitprecision point layouts can be defined among the 256 possibilities. Forany given generated message, only a single point layout need be used toencode all the historical position points (track), although the selectedpoint layout can change from message to message.

Accordingly, more recent history points can be defined with greaterprecision (e.g., 9-bit), while older history points can be defined withlesser precision (e.g., 5-bit). This may allow for additional historypoints to be included in the message.

In some cases, the last (oldest) history point may use a differentpredefined time duration, or even point layout. For example, in additionto the track history defined above, with, e.g., 6 historical positionsat 30 minute intervals for a 3 hour history, a final history point maybe added that is more distant in the past (e.g., 6 hours back) on afixed, large spaced point layout (which may have higher error). In someother cases, a variable time interval may be chosen, in which olderhistory points are progressively further apart. In still otherembodiments, variable time intervals may be chosen to increase temporalresolution closer to course changes and to decrease temporal resolutionwhen course is maintained relatively constant. In some embodiments,constant time offsets that are staggered between adjacent messages mayalso be used (e.g., using a 1-hour interval in a first message, with thetrack history offset by 30 minutes in each subsequent new message).

The point layout selection index values may be extracted from themessage sub-type identifier and converted to unsigned integers with themost significant bit being the first bit (e.g., 1010 0000 is 160).

For a regular message with a complete track history, the bit allocationfor the unencoded binary payload can be as shown in Table 2 below:

TABLE 2 Field Bits MMSI 30 Message sub-type identifier 9 Longitude 22Latitude 21 Course Over Ground 8 Burst Sequence Flag 1 Track History 45Payload CRC 16

In the example embodiments, the Longitude and Latitude fields containlongitude and latitude values for a current position report, asdetermined by an onboard GNSS receiver or other device. The currentposition may be the instantaneous position of the vessel being trackedat the time the message is generated. In other cases, the currentposition may be the most recent position or self-anticipated position atone of a number of prescribed time intervals within each hour of theday. For example, all positions may use the convention of the positionof the vessel in the most recent 10 minute mark from the start of thehour, i.e., 0:00:00, 0:10:00, 0:20:00, etc. (h:mm:ss), rather than thecurrent position. Such a method would produce positional and historicalvessel positions at synchronized times, regardless of the actual timeslot used to transmit the message, and would permit a higher fidelityrepresentation of relative vessel positions in a crowded environmentthan a combined chart showing vessels at staggered times (and hencepositions).

Longitude values may be stored as signed integers. With 22 bitprecision, this allows for representing values between −180 and +180 inincrements of 1/190 of a minute (with East as positive direction).

Similarly, latitude values may be stored as signed integers. With 21 bitprecision, this allows for representing values between −90 and +90 inincrements of 1/190 of a minute (with North as positive direction).

Other increments and coding are possible, for example if the availablebit depth is restricted or expanded.

The Course Over Ground may be an unsigned integer. With 8 bit precision,the COG may be delineated in 360/256 (i.e., 1.40625) degree increments.

The Burst Sequence Flag is a field (in the example embodiments, a 1-bitfield) that may be used to identify the current “burst sequence” orwindow for the current message. In the example e-Navigation messagetransmission protocol, messages are generated once per burst sequence,and then fixed for all subsequent retransmissions, every two minutes,within a burst sequence. This enhances the opportunity for a receiver tosuccessfully receive at least one message within the burst sequence, forexample, in a noisy environment. This also reduces the power andcomplexity requirements for the encoder, since there is no need torecomputed Track History for every re-transmission.

In some example embodiments, messages with a common Track History (i.e.,those within a single Burst Sequence) can be sent five times in a10-minute period, for example. Other implementations are also possible.

Accordingly, since the historical points are all at predetermined timeboundaries, even a message sent or received, for example, at 28 minutespast the hour can be known to come from a position fix taken at 20minutes after the hour (as is every messages at 22, 24, 26 and 28minutes).

As each of these multiple messages may be identical, reception can beimproved. However, if any one of the messages is decoded error free(e.g., the payload CRC check indicates a valid message), then repeatedmessages can be ignored. If, however, none of the CRC checks formultiple repeated messages, indicate a valid message, it maynevertheless be possible to successfully extract a message by applyingcombined decoding techniques.

For example, repeated messages may be non-coherently averaged or aweighted “voting” procedure used for each bit, to generate a compositemessage that may have fewer bit errors, and which can be successfullydecoded.

In practice, the Burst Sequence Flag is toggled at the beginning of eachnew burst sequence (e.g., every 10 minutes). That is, if the BurstSequence Flag is initially set to 0, it will switch to 1 after 10minutes have elapsed, and back to 0 after another 10 minutes. In someembodiments, the burst sequences may be aligned with time boundaries,such as 0, 10, 20, 30, 40 and 50 minutes past each hour.

Use of the Burst Sequence Flag prevents a possible message timing errorthat could occur if there are time differences between the AIStransceiver and the receiving satellite that cross a predefined timeboundary. Such differences can arise due to computation time required bythe AIS transceiver, signal propagation time, timing offsets between AIStransceiver and satellite, and errors in clocks used to time-stampreceived signals or messages, and otherwise.

In an example embodiment, the Burst Sequence Flag is toggled when thecurrent time reaches 0, 10, 20, 30, 40 and 50 minutes past each hour.

In the event of a received message, the Burst Sequence Flag can be usedto correctly associate the received message with a burst that wastransmitted in the correct 10 minute segment (correction of an error inthe received message time stamp of up to +/−5 minutes may berealizable).

Track History is described further elsewhere herein.

Payload CRC is a checksum for only the payload portion of the message.The Payload CRC may be generated using the same algorithm as the FullMessage CRC generated for the wrapper of the message. However, in otherembodiments, the Payload CRC may be generated using any other suitablealgorithm, and may comprise any number of bits. In the exampleembodiments, 16 bits are used.

A partial track history message may be provided anytime a track historycontains fewer than 5 track history points for a 9-bit precision messagepoint layout, or fewer than 9 track history points for a 5-bit precisionpoint layout. This may occur, for example, if a transceiver has beenrecently powered on, or if other data is desired to be included in themessage (e.g., alert flags). If the bit precision of a point layout isother than 5 or 9 bits, or the bit precision changes with historicaltime, partial track histories can be used when the available trackhistory (e.g., in memory 270) is shorter than the number of pointsrequired to fill the track history report.

For a partial track history message, the bit allocation for theunencoded binary payload can be as shown in Table 3 below for 5-bitprecision:

TABLE 3 Field Bits MMSI 30 Message sub-type identifier 14 Longitude 22Latitude 21 Course Over Ground 8 Burst Sequence Flag 1 Track History 5to 40 Spare Variable Alert Flags (optional) 0 or 5 CRC 16

For a partial track history message, the bit allocation for theunencoded binary payload can be as shown in Table 4 below for 9-bitprecision:

TABLE 4 Field Bits MMSI 30 Message sub-type identifier 13 Longitude 22Latitude 21 Course Over Ground 8 Burst Sequence Flag 1 Track History 9to 36 Spare Variable Alert Flags (optional) 0 or 5 CRC 16

As noted above, when an incomplete or partial track history is present,the message sub-type identifier may be up to 14 bits in length. For a5-bit precision message, the message sub-type identifier may be 14 bitsin length. For a 9-bit precision message, the message sub-typeidentifier may be 13 bits in length.

The first two bits of the message sub-type identifier can be set to 0,to distinguish over a complete track history message.

As with a regular message with complete track history, the next 8 bitsmay be a point layout selection index to select the appropriatepredetermined point layout for the track history contained in themessage. These indices and point layouts may be the same as those usedin a full track history.

If the selected precision is 9-bit, the next 3 bits of the messagesub-type identifier represent the number of track history points createdfor the partial track history contained in the message (i.e., 0 to 4),resulting in a total length of 13 bits for the message sub-typeidentifier. Similarly, if the selected precision is 5-bit, the next 4bits of the message sub-type identifier represent the number of trackhistory points created for the partial track history contained in themessage (i.e., 0 to 8), for a total length of 14 bits.

The Spare field can be used to pad or bitstuff the payload to maintain afixed length. Generally, the length of the Spare field will bedetermined by the number of track history points in the partial historymessage, and whether or not an alert is set.

For example, for a 5-bit precision message, the length of the Sparefield can be determined by the following formula:

L=(A+7−N)*5

where L is the length in bits, N is the number of track history points.A is 0 if an alert is set, and 1 otherwise.

Similarly, for a 9-bit precision message, the length of the Spare fieldcan be determined by the following formula:

L=(4−N)*9+A*5

where L is the length in bits, N is the number of track history points.A is 0 if an alert is set, and 1 otherwise.

Although the described embodiments refer to 5-bit and 9-bit precision,other bit precisions are possible. For example, a 4-bit precision or8-bit precision could also be used, with appropriate modification of thedescribed embodiments. A partial track history message can also beselected even when there are sufficient history points in memory, topermit usage of above spare bits for additional information, asdescribed in further detail below.

The Alert Flags field may be used for partial track histories that have7 or fewer track history points for 5-bit precision messages or thathave 4 or fewer track history points for 9-bit precision message. Ifmore track history points are present, the alerts may be unset oromitted. If an alert flag is set when there are 8 or more track historypoints (for 5-bit history) or 5 track history points (for 9-bithistory), the message sub-type identifier can indicate that the numberof track history points reported is 7 or 4 (depending on the pointlayout bit depth) and include the alert flags field with the appropriateflags set.

In some embodiments, the Alert Flags field may be made larger,particularly where different bit precisions are used for the TrackHistory. For example, with 45 bits available in the Track History field,if a bit depth of only 8 bits is used, only 40 of 45 bits can be usedfor Track History. Accordingly, the remaining bits of the Track Historyfield can be allocated for use as alert flags.

Similarly, the Spare field can also be allocated for use as alert flagsin some embodiments.

Alert Flags are toggle bits that may be used to indicate the alertstatus of the transmitting device. Multiple alerts can occursimultaneously; therefore a bit pattern is used to transmit the completealert status with each message.

If no alert flags are set the alert status need not be sent and theAlert Flags field can be allocated for use in providing Track History.

An example definition of the alert bits is shown below in Table 5:

TABLE 5 Alert bit Description 0 SOS alert 1 Geofence alert 2 Bracketalert 3 Power alert 4 Reserved for future use

Alert bit 0 may indicate the SOS status of the vessel (i.e., SOS activeor inactive). Alert bit 1 may indicate whether the vessel is outside ageofence that may be predefined for the AIS transceiver. Alert bit 2 mayindicate a mounting bracket alert. Alert bit 3 may indicate a low powercondition. Alert bit 4 may be reserved for future use.

A bracket alert can indicate a potential security condition. In somecases, transmitters are placed in a dedicated mounting bracket while inuse. This bracket may be locked to a specific transmitter (e.g., by aRFID tag or other “keying” device). If the transmitter is removed, orinstalled in a different bracket, it can be configured to set an alertthat indicates this condition.

Other combinations of alert bits are possible. In some embodiments, theAlert Flags field may be enlarged, allowing for still other alert bitsto be provided, or for more detailed messages.

In some embodiments, additional flags may be possible to convey AIStransceiver health information (e.g., temperature, battery voltage,firmware version, uptime, etc.), local weather, or other information.

Track History

In the example embodiments, the Track History field will typically havea maximum length of 45 bits, due to limitations in the length of atwo-slot AIS message. However, in other embodiments, additional slots,or partial slots may be used to provide additional history data, orgreater precision of the history points in time or position.

Several bit precisions are demonstrated in the example embodiments: onein which the history is provided in up to 5 samples with 9-bitprecision, and one in which the history is provided in up to 9 sampleswith 5-bit precision.

Each sample may be an approximation of a previous transceiver location,placed on a predefined point layout which will be known to the AISsatellite receiver (or ground processing station) and can be identifiedfrom the message sub-type identifier. The points in a point layout neednot be spaced in a Cartesian grid coordinate system. Rather, radialcoordinates and other coordinate systems may be used. For example, insome embodiments, the predefined point layout may contain predefinedpoints spaced radially, and identified according to radial and angularcomponents. Still other predefined point layouts can be used, such aspredefined shapes, equally sampled per linear length of fractal numbers,discrete cosine transforms (DCT) or Fast Fourier transforms (FFT)layouts based on transformed encodings of ‘elemental’ shapes, such asstraight lines, various curves, ellipses and circles, etc.

Although the examples provided herein generally use the examples ofpoint layouts in which points are spaced according to some ordered oralgorithmic approach (e.g., Cartesian grid, concentric circles, etc.),in other cases, there may be point layouts in which points are placedarbitrarily. FIG. 9 illustrates one such arbitrary point layout, inwhich points are placed according to some automatic or manual tuningprocess. The arbitrarily placed points can be identified by index values(i.e., a first dimensional component value array index) in similarfashion to the ordered or algorithmically derived point layouts.

In such arbitrarily defined point layouts, it is possible to assign afixed single point to correspond to no motion, or no change in motionfrom a previous encoded historical point, or to encode special cases,such as the presence of an alert flag. When used in conjunction withalert flags as described above, particular points may be used toindicate at which point in a history the alert had occurred (to withinthe temporal resolution of the history report). An alternate encodingcould use several dedicated points to indicate two or more unique alertstatus bits within the history report. Both of the latter approacheswould provide some temporal information context to alert flags, and mayavoid the need for a partial history report to insert an alert flag.

In some cases, the arbitrarily defined point layouts may be combinedwith ordered or algorithmically defined point layouts to allow theencoding of alert flag tags. For example, a predefined point layout maysacrifice one bit of precision at each point in the layout to be used asan alert flag tag. That is, rather than using 9-bits of precision, only8-bits may be used, with the remaining 1-bit used as a tag to indicatewhether the alert flag applies at that history point.

The exact numerical components of each of the points in a point layoutmay be defined in a point layout look-up table or tables shared by theAIS transceiver and AIS satellite receiver (or ground processingstation), and referenced according to index values for each table entry.

In an example embodiment with 5-bit precision and a radial pointarrangement, the radial component may have 2 bits, which allows for 4potential component values, and the angular component may have 3 bits,which allows for 8 potential component values.

In another example embodiment with 9-bit precision and a radial pointarrangement, there may be a variable number of bits ranging from 3 to 6bits for each of the radial and angular components. The number of bitsused for each component may be defined by the specific look-up tableused, however, the number of bits per component is fixed within eachindividual look-up table.

Referring now to FIG. 5, there is illustrated an example point layout500 for a radial point arrangement with 5-bit precision. In the examplepoint layout, the angular components are equally spaced. With 3-bitprecision for the angular component, up to 8 different angularcomponents are possible and with equal angular spacing, each of theseangular components is 45° from its nearest neighbors. In some otherembodiments, variable angular spacing may be used.

The radial components in point layout 500 may be variably or equallyspaced. With 2-bit precision for the radial component, only 4 possibleradial lengths are possible and in the point layout 500, these areselected as 1, 2, 4 and 8 nautical miles. Various other radial lengthsare possible, and examples of other radial component values are shown inTable 7.

The combination of 4 possible radial components and 8 possible angularcomponents combines for up to 32 predefined points 510.

When generating a track history, the currently reported position of theAIS transceiver is chosen as the origin point 505 of the point layout.In the example of point layout 500, this may be the position with a zeroradial component. In a Cartesian grid layout, for example, the originpoint may be a point with zero value x and y components.

A most recent track history point may be identified next. This mostrecent point may be identified from a locally-stored log of AIStransceiver positions in a predefined time period. In some embodiments,the most recent point may be identified from a prior AIS message, oreven a prior track history position.

In general, the most recent track history point may be determined byfinding the predefined point in a predefined point layout that mostclosely approximates the location of the AIS transceiver at a known timein the past (e.g., along a time boundary, such as on the hour).

In other examples, relative time periods may be selected as 10 or 30minutes into the past, based on the current time of the message.Accordingly, the most recent track history point will be determined byidentifying the predefined point closest to the actual position of theAIS transceiver at either 10 or 30 minutes in the past.

To reduce timing ambiguity, and to reduce the number of bits needed torepresent track history points, a requirement may be imposed that onlythose position reports transmitted along fixed time boundaries, relativeto Universal Coordinated Time (UTC) may be used. Thus, the currentposition may be determined every 10 minutes, relative to UTC, and a newtrack history computed. Points in the track history may be those pointsthat fall on 30 minute boundaries, relative to UTC. All 10 and 30 minuteboundaries may be relative to UTC time.

Since there may be 5 identical messages sent within each 10 minuteperiod (e.g., Burst Sequence), the time from the current time in thegenerated message to the first track history point can be either 10, 20or 30 minutes. For example, if the original message in a Burst Sequencefalls on a 30 minute boundary, the most recent track history point willbe 30 minutes into the past. If the original message in a Burst Sequencefalls on a 10- or 20-minute boundary, then the most recent track historypoint will be 10- or 20-minutes in the past, respectively.

Generally, the first message in a Burst Sequence may be transmitted inthe first time slot that falls within a 10-minute boundary (e.g.,between 00:00 and 02:00 minutes of a 10-minute window). The nexttransmission would then occur in the next time slot (e.g., between 02:00and 04:00 of the 10-minute window). In some cases, a GNSS fix may takesome time to obtain, which could result in the time slot for theexpected transmission to be missed. For example, if a GNSS fix requires5 seconds to complete, but the time slot falls at 00:01 after the10-minute boundary, the completed fix may not be available when themessage should be sent. Accordingly, position fixes may be predeterminedin advance of a 10-minute boundary, so that the message is availablewhen needed for transmission.

In order to maximize channel capacity, a time slot randomization routinemay be used within each two minute window, resulting in a form of randomaccess time division multiple access (RA-TDMA). For example, eachtransmitter may employ an algorithm to select the time slot to be used,in which the selected time slot is determined based on the MMSI of thetransmitter. In subsequent two-minute windows, different time slots maybe selected, again based on the MMSI, thus avoiding the problem ofalways selecting the same time slot as a nearby transmitter and theresulting signal collisions. Various approaches can be used to providerandomization, such as using the last digit(s) or performing some formof CRC check on the MMSI to determine an amount of slots to jump.

Table 6 illustrates the relation between message time and track historypoint times for a one hour period beginning at 12:00 UTC, on one exampleembodiment.

TABLE 6 Message Time 12:00 12:10 12:20 12:30 12:40 12:50 Track History 111:30  12:00 12:00 12:00 12:30 12:30 Track History 2 11:00  11:30 11:3011:30 12:00 12:00 Track History 3 10:30  11:00 11:00 11:00 11:30 11:30Track History 4 10:00  10:30 10:30 10:30 11:00 11:00 Track History 59:30 10:00 10:00 10:00 10:30 10:30 Track History 6 9:00  9:30  9:30 9:30 10:00 10:00 Track History 7 8:30  9:00  9:00  9:00  9:30  9:30Track History 8 8:00  8:30  8:30  8:30  9:00  9:00 Track History 9 7:30 8:00  8:00  8:00  8:30  8:30

It will be appreciated that various other time intervals or timeboundaries may be used. In some alternative embodiments, timinginformation may be incorporated into the binary payload itself.

For example, in another approach, the time offset for the track historymay alternate between neighboring Burst Sequences. For example, BurstSequences that begin at 0, 20 and 40 minutes past each hour may containtrack history points determined on the hour, and at one hour intervals.Conversely, Burst Sequences that begin at 10, 30 and 50 minutes pasteach hour may contain track history points determined on the midpoint ofeach hour, also at one hour intervals. In this way, successful receptionof reports from a vessel in just two consecutive BurstSequences—potentially as little as two minutes apart—could double thedata points available for a vessel's track history. Moreover, even ifonly a single position report is received, adequate track history isnevertheless available. An example of this approach is illustrated inTable 7.

TABLE 7 Message Time 12:00 12:10 12:20 12:30 12:40 12:50 Track History 111:00  11:30  11:00  11:30  12:00  12:30  Track History 2 10:00  10:30 10:00  10:30  11:00  11:30  Track History 3 9:00 9:30 9:00 9:30 10:00 10:30  Track History 4 8:00 8:30 8:00 8:30 9:00 9:30 Track History 57:00 7:30 7:00 7:30 8:00 8:30 Track History 6 6:00 6:30 6:00 6:30 7:007:30 Track History 7 5:00 5:30 5:00 5:30 6:00 6:30 Track History 8 4:004:30 4:00 4:30 5:00 5:30 Track History 9 3:00 3:30 3:00 3:30 4:00 4:30

In some embodiments, the required code space (in bits) for the TrackHistory may be reduced by employing Huffman coding or other equivalentapproaches for frequently occurring words (e.g., positions, indexes,etc.).

A Huffman code may be used, for example, to encode the Track Historyusing a variable number of bits per history point, with more frequentlyoccurring encodings using less bits, and less frequently used historypoints using more bits, resulting in a net reduction in the number ofbits required to encode an average track history. Such economy can beemployed empirically when the historical position behavior results in anencoded track history of shorter average duration (less bits) to add oneor more additional historical positions to this message. This processcan happen dynamically, providing a variable length message that alwaysuses available or allocated bits for history reports to the maximumextent possible, and when unusual behavior increases the average numberof bits per position report in a particular message, the time durationof only that message can be shortened to fit the available bits.

For example, consider a circle with several diameters representing theradium, and 8 points around each radius representing direction. In thismapping, motion since the previous time interval, e.g., straight aheadis defined in an upward direction (i.e., toward the top of the circle).The points at the extreme left and right side are the least likely tooccur, as they would only be used for a vessel at high speed, which isalso making a 90° turn.

If a Huffman code is used, the most common motion, e.g., straight ahead,or some minor course change, or slow movement with larger angularchange, may use less bits, while the extreme motion points above woulduse more encoding bits as these are expected to be more rare.

The described embodiments generally encode history points as acoordinate in space (e.g., radius and angle, x- and y-component, etc.).However, in some other embodiments, history points could be encoded as avelocity (a derivative of position) or an acceleration (a secondderivative of position). Since the time interval is generally heldconstant between history points, both average velocity and averageacceleration can be determined from change in position. The reverseapproach could also be used if velocity or acceleration are encodedinstead. Such encoding of changes in velocity for example may result inmore efficient and higher precision encoding of historical positions forvessels that are not moving, or alternately continue to move at the samespeed and heading (i.e., not changing their behavior). The latterscenario may enable efficiency gains when further combined with Huffmancoding, described above.

Referring now to FIG. 6, there is illustrated a point layout 600. Pointlayout 600 follows the same predefined point pattern shown in FIG. 5. Acurrent location point 605 is shown at the origin point of point layout600. The actual historical position of the AIS transceiver at a time 30minutes in the past is shown at actual historical point 620. The nearestpredefined point to actual historical position point 620 is predefinedpoint 630. Accordingly, predefined point 630 is selected as the mostrecent track history point.

To represent the most recent track history point in the Track Historyfield, the index values for each of the radial and angular components ofpredefined point 630 may be concatenated in binary form. For example,predefined point 630 is at the 8^(th) angular position from a Northcompass point position when counted in clockwise order, accordingly, the3-bits representing angular component are 111 in binary form (i.e.,8−1=7). Similarly, the radial component is at 4 nautical miles, or thethird radial length, leading to 2-bits representing the radial componentas 10 in binary form (i.e., 3−1=2). The 5-bit representation ofpredefined point 630 may be 11110, when angular component is specifiedfirst.

Additional track history points can be computed in similar fashion, bydetermining the nearest predefined points to previous historicalpositions separated by a fixed or otherwise predetermined time interval.Each additional track history point can be concatenated in the TrackHistory field, up to a maximum Track History length.

Preferably, each predefined point in the Track History is recomputedwhen a new e-Navigation message is generated, since the origin point mayshift (i.e., due to a new current location), causing previously selectedpredefined points to be less accurate representations of the historicalpositions stored in the AIS transceiver memory. However, merere-transmissions of an e-Navigation message may not require recomputingpoints, since the origin point remains the same within the same BurstSequence window.

Moreover, when the Track History is recomputed, a different predefinedpoint layout may be used, and even a different bit precision or numberof historical points. As such, each Track History has independentencoding estimation errors (which may also be referred to as“quantization” artifacts). Accordingly, a receiver of severale-Navigation messages from a particular AIS transceiver that containdifferent Track History fields (i.e., not simply re-transmissions withina Burst Sequence) with points that overlap in time, may use some form ofstatistical averaging of the reported historical positions at the samepoint in time to reduce the uncertainty of position in that same point(e.g., reduce the effective quantization error between the calculatedand averaged historical position and the true unquantized position,known only to the encoder).

In order to reduce error in representing historical positions, variousdifferent radial and angular point layouts may be predefined inpredefined tables. Moreover, different component values for pointlayouts may be predefined in component value arrays. This dualflexibility in both the relative arrangement of points, and also thespecific component values assigned to points, allows for a lowest errorrepresentation to be constructed.

A best fit may be performed by computing the error for history pointsusing a selection of predefined component values. The specificpredefined component values chosen may be those that provide the lowesterror between historical positions and track history positions. Variousapproaches may be used to minimize “quantization” error between thehistory points and the predefined points. For example, optimizationroutines may be based on the maximum error (e.g., in nautical miles orkilometers), total error in a Track History (e.g., the sum of all radialerrors for all history points), root mean square error over all historypoints, and others. In some embodiments, combinations of theoptimization routines may be used, or time-weighted criteria may be used(e.g., provide greater accuracy for more recent history points).

Generally, the selection of point layout, and optimization routines, maybe based on known or expected behavior of a vessel. For example, in railcar applications, point layouts may be designed to contain more pointsin a forward or backward direction of motion, since rail cars generallytravel along a fixed path with relatively radius turns. Likewise, anocean-going vessel at a cruising speed, or an aircraft at cruisealtitude, may favor point layouts with points clustered along aparticular axis.

In some embodiments, the selected fit may be based on one or morequality metrics, such as average or absolute error. For example,selected fit may be “fit with minimum mean square error and also no morethan 1 km of maximum radial error for any history point”. In someembodiments, verification may be performed that eliminates fits thatinclude history points that would appear to be on land (if the vessel isnot actually on land, and a map of land contours is available to theencoding process).

Once the lowest error is determined and the specific predefinedcomponent values selected, the chosen predefined tables may beidentified in the message sub-type identifier. In some exampleembodiments, various different combinations of first dimensionalcomponent values (e.g., radial) and second dimensional component values(e.g., angular) are defined in respective arrays of component values.Corresponding point layout look-up tables contain combinations of firstdimensional indices and second dimensional indices, allowing for aspecific combination to be identified by a compact point layout index.

In other embodiments, for example where point layouts with arbitrarypoint placement are used, a second dimensional component value may beomitted, and only a single, first dimensional component value thatidentifies a particular point (e.g., by referencing coordinate pairs ina predefined table) may be used.

Example predefined component value arrays, which specify points in aradial arrangement, are shown below. In the examples, all radialdistances are in nautical miles. Angular measures are in degrees fromthe Course Over Ground direction (measured clockwise) of the currentposition of the AIS transceiver, or from the previous track. In someother embodiments, Angular measures may be in degrees from true north,or some other reference.

TABLE 7 R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 0.05 0.25 0.05 6 0.1 0.250.25 0.05 0.05 0.25 0.05 0.025 0.5 1.5 0.5 8 0.75 2 6 0.5 1 0.50 0.50.25 1 3 1 10 1.5 4 8 1 2 0.75 1 0.50 2 4.5 12 12 2.25 6 10 1.5 3 1.001.5 0.75 3 6 14 14 3 8 12 2 4 1.25 2 1 4 7.5 16 16 4 10 14 2.5 5 1.502.5 1.25 5 9 18 18 5 12 17 3 6 1.75 3 1.50 6 11 20 20 6.5 14 20 3.5 72.00 3.5 1.75 4 8 2.25 4 2 4.5 9 2.50 4.5 2.25 5 10 2.75 5 2.50 5.5 113.00 5.5 2.75 6 12 3.50 6 3 6.5 13 4.00 6.5 3.25 7 15 4.50 7 3.50 7.5 185.00 7.5 3.75 8 4 8.5 4.25 9 4.50 9.5 4.75 10 5 10.5 5.25 11 5.50 11.55.75 12 6 12.5 6.25 13 6.50 14 7 15 7.50 16 8 18 9 20 10

Table 7 illustrates twelve separate component value arrays, labeled R1through R12, which contain between 8 and 32 possible values. Dependingon the amount of bits available to represent the radial component, notall arrays may be available (e.g., if only 3 bits are used, then R8through R12 may not be available). Each of the values has an array indexvalue. In practice, each e-Navigation message will identify the specificcomponent array to be used, and the index of the component value to beused.

For example, an e-Navigation message may represent a radial component of5.25 NM by specifying that radial component array R12 is to be used, andproviding an index of 21 (10101). This uses only 4 bits to specify thecomponent array, and only 5 bits to represent a specific value (5.25).Additional values for additional track history points can simply referto other values within the same component array. For example, anotherhistory point may have a radial component value of 0.75 NM, which can bereferenced at index 3 (00011).

Angular components can be represented in similar fashion. Table 8illustrates eight separate component value arrays, labeled T1 throughT7, which contain between 16 and 32 possible values. Depending on theamount of bits available to represent the angular component, not allarrays may be available (e.g., if only 3 bits are used, then T1 throughT5 may not be available). Each of the values has an array index value.In practice, each e-Navigation message will identify the specificcomponent array to be used, and the index of the component value to beused.

For example, an e-Navigation message may represent an angular componentof 110.76° by specifying that angular component array T4 is to be used,and providing an index of 6 (00110). This uses only 3 bits to specifythe component array, and only 5 bits to represent a specific value(110.76). Additional values for additional track history points cansimply refer to other values within the same angular component array.For example, another history point may have an angular component valueof 0°, which can be referenced at index 0 (00000).

TABLE 8 T1 T2 T3 T4 T5 T6 T7 0.000 0 0 0 3 0 4.28 5.625 45 11.25 45 9 5012.85 11.250 90 22.5 67.50 15 100 21.42 16.875 93 33.75 90 21 113.33 3022.500 96 45 96.92 27 126.66 150 28.125 99 56.25 103.84 33 140 158.5733.750 102 67.5 110.76 39 153.33 167.14 39.375 105 78.75 117.69 45166.66 175.71 45.000 108 90 124.61 135 180 184.28 50.625 111 101.25131.53 141 193.33 192.85 56.250 114 112.5 138.46 147 206.66 201.4261.875 117 123.75 145.38 153 220 210 67.500 120 135 152.30 159 233.33330 73.125 123 146.25 159.23 165 246.66 338.57 78.750 126 157.5 166.15171 260 347.14 84.375 129 168.75 173.07 177 310 355.71 90.000 132 180180 183 95.625 135 191.25 186.92 189 101.250 138 202.5 193.84 195106.875 141 213.75 200.76 201 112.500 144 225 207.69 207 118.125 147236.25 214.61 213 123.750 150 247.5 221.53 219 129.375 153 258.75 228.46225 135.000 156 270 235.38 315 140.625 159 281.25 242.30 321 146.250 162292.5 249.23 327 151.875 165 303.75 256.15 333 157.500 168 315 263.07339 163.125 171 326.25 270 345 168.750 174 337.5 292.50 351 174.375 177348.75 315 357 180.000 180 185.625 183 191.250 186 196.875 189 202.500192 208.125 195 213.750 198 219.375 201 225.000 204 230.625 207 236.250210 241.875 213 247.500 216 253.125 219 258.750 222 264.375 225 270.000228 275.625 231 281.250 234 286.875 237 292.500 240 298.125 243 303.750246 309.375 249 315.000 252 320.625 255 326.250 258 331.875 261 337.500264 343.125 267 348.750 270 354.375 315

In some embodiments, the representation of component value arrays can bemade even more compact, by predefining specific combinations of firstand second dimensional component value arrays, and using indexes torefer to these specific combinations. This index may serve as theeffective first dimensional component value in such embodiments, and thesecond dimensional component value can be omitted from the message.Table 9 provides an example point layout look-up table that providessuch combinations. An index column identifies indexes for eachcombination, which refers to radial component arrays R1 through R12 (asshown in Table 7) and angular component arrays T1 through T7 (as shownin Table 8).

TABLE 9 Index Radial Angular 0 Undefined Undefined 1 R1 T1 2 R2 T1 3 R3T1 4 R4 T1 5 R5 T2 6 R2 T2 7 R6 T2 8 R7 T2 9 R8 T3 10 R9 T3 11 R10 T3 12R8 T4 13 R9 T4 14 R10 T4 15 R8 T5 16 R9 T5 17 R11 T6 18 R12 T7 19 R11 T620 R12 T7 21-255 Future use Future use

When Course Over Ground direction is used as the reference for angularmeasures, the Course Over Ground reference may change from onee-Navigation to the next, because vessels typically will not keep thesame course for several hours. In some embodiments, to minimize error,the most recent track history point may be the only point whose angularcomponent is measured with respect to the Course Over Ground reference.Subsequent track history points may use an angular component calculatedwith reference to a line that intersects the two previous points. Thismay be referred to as a calculated or simulated Course Over Ground. Thesimulated Course Over Ground for each point may be used as the reference(i.e., determine 0°) for the purpose of determining an angular measurefor the next track history point.

Locations provided by a GNSS receiver are typically provided in latitudeand longitude. In order to convert latitude and longitude to radial andangular components, a conversion can be applied. Since lines of latitudedo not converge, 1 nautical mile may be defined as always approximatedby 1/60 of a degree. Therefore the conversion from latitude to distancecan be determined from:

d _(lat)=Δlat*60

where d_(lat) is the distance in nautical miles in the y-axis of theunrotated GNSS grid, and where Δlat is the change in points of latitude(in degrees).

Lines of longitude converge at the North and South Poles. Accordingly,the distance between lines of longitude changes with respect tolatitude. To map measured longitude with the track history pointsspecified in radial and angular components, a mapping function may beused, which uses a multiplier to account for the varying distances inlongitude depending on changes in latitude.

In some examples, a constant multiplier value may be used for eachtransmitted message.

The constant multiplier can be calculated using the most recentlyupdated latitude. Using a spherical Earth approximation, the conversionfrom longitude to distance will be:

d _(lon)=(2*pi*R _(E)*cos(lat))*(Δlon/360)

Where d_(lon) is distance in nautical miles in the x-axis of theunrotated GNSS grid, R_(E) is approximate Earth equatorial radius of3444 nautical miles, lat is the latitude in the message to betransmitted, and Δlon is the change in points of longitude (in degrees).

The factor d_(lon) may be used for conversions from longitude todistance for the message to be transmitted. Use of d_(lon) effectivelyresults in an Oblique Gnomic projection of the Earth.

Referring now to FIG. 7, there is illustrated a flow diagram for anexample method 700 of generating an e-Navigation message. Method 700 maybe carried out, for example, by transmitter 200 of FIG. 2.

As described herein, the generated e-Navigation message can contain abinary payload. The binary payload can be encapsulated in a messagewrapper that is compliant with industry standard messaging formats, suchas AIS. In addition, one or more error protection (which may also becalled error correction or error recovery) encoding schemes can beapplied to the binary payload in order to maximize the probability ofdetecting this portion of the message at the receiver. In particular, aForward Error Correction (FEC) coding may be applied.

In some embodiments, method 700 may begin with the transmitter 200waiting for a timing boundary at 705, such as, for example, a 10 minuteboundary or some other predefined boundary that delineates the beginningof a new Burst Sequence. Once the timing boundary is encountered, aBurst Sequence Flag value may be changed to indicate the start of a newBurst Sequence. In some other embodiments, if a timing boundary is notused, method 700 may begin at 710 or after some predefined fixedinterval.

At 710, a position module or GNSS receiver determines a currentgeolocation of the vessel or alternately, has a recent position fix thatcan be read from a memory location or register, its GNSS receiver or theAIS transceiver. If a standalone GNSS receiver is used, the GNSSreceiver may output the current geolocation to the AIS transceiver. Thecurrent or recent geolocation generally includes a current longitude andlatitude of the GNSS receiver. The AIS transceiver or GNSS receiver mayfurther determine a Course Over Ground of the vessel. In an exampleembodiment, a position fix and a COG obtained less than about 30 seconds(preferably less than 10 seconds) before the time boundary should beadequate to minimize discrepancies between reported and actual positionat a time boundary.

At 712, the AIS transceiver may generate portions of the e-Navigationmessage wrapper, such as Message ID, Repeat Indicator, MMSI, andApplication ID.

At 715, the AIS transceiver determines one or more e-Navigation messageparameters. Examples of parameters to be determined include, forexample, a transmitter identifier and a portion of the message sub-typeidentifier—which may itself involve determining whether a message to begenerated will include a full track history or a partial track history.In the case of a partial track history, the message parameters mayfurther include a number of track history points to be included in themessage, Alert Flags to be included in the message, and a Spare fieldvalue.

Once the number of track history points is known, sufficient historicaltrack history points may be retrieved from a memory of the AIStransceiver at 720. Historical track history points may be retrievedfrom a locally-stored log of prior e-Navigation messages (e.g., based onthe current locations included in those messages) or from stored GNSSdata. As described herein, historical track history points may beseparated by a fixed interval of, for example, 30 minutes.

At 725, AIS transceiver may iterate through a plurality of predefinedpoint layouts in one or more predefined point layout look-up tables, toidentify a best fit of the historical track history points to one of thepredefined point layouts. The chosen predefined point layouts can beidentified by their associated indices in the predefined look-up tables.

In other embodiments, specific look-up tables for each of the first andsecond dimensions may be used. For example, a first table with a firstindex may identify the chosen component values in a first dimension,such as radial component or x-component, and a second index may identifythe chosen component values in a second dimension, such as angularcomponent or y-component.

In some embodiments, the AIS transceiver may select longer trackhistories (i.e., with less bit precision) for less complex tracks, suchas where there are no course changes over a multi-hour period.Conversely, the AIS transceiver may select shorter track histories(i.e., with greater bit precision) for more complex tracks, such aswhere there have been one or more course changes over a multi-hourperiod. This allows for a trade-off between accuracy and history length.In some cases, the AIS transceiver may allow for lesser accuracy (e.g.,greater error) to allow for longer history. In other cases, the AIStransceiver may always seek to minimize error.

For example, the AIS transceiver may analyze possible point layouts todetermine whether multiple point layouts can provide a maximum thresholderror of, e.g., 1 km of error at any of the history points. If so, thepoint layout that uses the minimum number of bits per point would beselected, even though another point layout may achieve a smaller amountof error but at a higher bit cost, and hence, a shorter number ofhistory points.

Once the point layout and component values are known, the messagesub-type identifier may be completed at 730.

At 735, the track history is encoded using the historical track historypoints and fitting to the selected predefined point layout and componentvalues, as described herein.

Once the track history is encoded, the binary payload is completed at740, by inserting Alert Flags or Spare field values, if any, at 740. Apayload CRC is also computed once the binary payload is fully specified.

In the example embodiments, the binary payload, including the payloadCRC, is error protection encoded, e.g., using forward error correctioncoding, at 745. The error protection encoded binary payload is thenencapsulated in a message wrapper as described herein to form thecompleted e-Navigation message, using message formatting module 265, forexample. For example, the message wrapper may be an AIS message wrapperincluding a ramp up field, an AIS training sequence, a start flag, anapplication identifier, a MMSI, a full message CRC computed for themessage, a buffer, and a stop flag.

The completed e-Navigation message is then transmitted at 760 using atransmitter such as transmitter 210 of FIG. 2. In some embodiments,transmission may occur over existing AIS frequencies (e.g., AIS channels1 or 2). In other embodiments where backwards compatibility with AIS isnot needed, new or dedicated frequencies may be used. The transmittermay then wait a predetermined interval, such as 2 minutes, at 765.

At 770, the AIS transceiver determines whether to the next BurstSequence time boundary has been reached. If the next time boundary hasbeen reached, the AIS transceiver returns to 710 to determine or utilizeeither a new or saved position from memory and generate a newe-Navigation message.

If the next time boundary has not yet been reached, the AIS transceivercan retransmit the previously generated e-Navigation message and returnsto 760 to re-transmit the message.

In some alternative embodiments, acts of method 700 may be omitted. Forexample, forward error correction may be omitted, or only one messagemay be sent per burst interval.

Referring now to FIG. 8, there is illustrated a flow diagram for anexample method 800 of receiving and processing an e-Navigation message.Method 800 may be carried out, for example, by receiver 300 of FIG. 3.

Method 800 begins at 805 by receiving a signal and generatingpre-processed signal data, using at least one antenna 310 and RFreceiver stage 320 of receiver 300, for example.

At 810, the pre-processed signal data is processed by message processingmodule 360, for example, which may perform GMSK demodulation followed byNon-Return to Zero Inverted (NRZI) decoding, de-stuffing and bitreversal to recover input signal data.

At 820, a candidate e-Navigation message may be identified in the inputsignal data. Candidate messages may be identified, for example, bycorrelating the input signal data with predetermined sequences. Forexample, the standard 24-bit AIS training sequence may be used toperform correlations, and optionally the 8-bit start flag to form atotal of a 32-bit correlation sequence. Other known sequences in AISmessages, such as the start flag and end flag, or AIS message type, mayalso be used. In some embodiments, still other fields or bit sequencesof the e-Navigation message—and the message wrapper in particular—may beused when they are known by the receiver or processor. For example, theMessage Type field may be used in some cases, when it is known by theprocessor. Various other techniques for identifying candidate messagesmay also be used.

In some cases, identification of candidate messages may exploit one morediversity schemes implemented at the transmitter, in addition to thosedescribed herein.

In conventional AIS receivers, once a candidate message is identified,the full message CRC may be used to validate the message data. Forexample, a CRC validation may be used to determine whether the messagedata contains one or more bit errors. In conventional receivers, thedetection of bit errors generally causes the receiver to discard themessage data. However, in the described embodiments, processing of thecandidate message may continue even in the presence of one or more biterrors.

Once a candidate message is identified, the binary payload can beextracted at 830. In some embodiments, the binary payload may bedetermined by computing one or more bit offsets from one or morepredetermined features (e.g., training sequence, start flag, messagetype, end flag, etc.) of the candidate message. For example, a bitoffset from the AIS training sequence may be used to identify the startof the binary payload and the payload may be extracted based on anexpected message length or by extracting data until a payload CRC, fullmessage CRC or message end flag is found, for example.

Once the error protection encoded binary payload is extracted from thecandidate message, error protection decoding may be attempted at 840.Decoding generally involves reversal of the encoding actions performedby the transmitter (e.g., transmitter 200). For example, if the errorprotection encoding comprised forward error correction encoding, thenthe decoding comprises forward error correction decoding.

At 850, it is determined whether the error protection was successfullydecoded. If error protection decoding fails (i.e., the error protectionencoding cannot be successfully decoded), or if CRC checks fail, the AISreceiver may store a candidate message which has failed validation at860. Otherwise, the AIS transceiver may proceed with retrieving messagedata from the decoded message at 890.

The AIS transceiver may determine, at 870, whether more than one failedmessage has been stored that belongs to a common Burst Sequence. If aplurality of stored messages are available, the AIS receiver may performrecovery processing at 880 as described herein (e.g., voting, summing,etc.), and generate a recovered message, which becomes the new candidatemessage for evaluation at 850.

As noted above, following decoding of the error protection scheme orschemes, the message data may be retrieved at 890. Retrieval of messagedata generally includes performing operations to reverse the sourcecoding applied by the transmitter when generating the raw binarypayload.

Accordingly, the described embodiments may, in some cases, provide asignificant overall enhancement to detectability of e-Navigationmessages in noisy and congested channels, and to provide ancillaryinformation such as track history in a compact representation.

In some alternative embodiments, the binary payload may not beerror-protection encoded, or the payload may not be binary encoded, orboth. In such embodiments, processing of the message may omit extractionof the binary payload, error-protection decoding or other acts of method800.

To facilitate the described detection approaches, correspondingtransmitters can be easily incorporated into new or existing AIStransponders. Existing AIS transponders may require changes to internalmessage assembly and coding algorithms. Additionally, message frequencyand co-ordination (scheduling) may need to be programmed in the languageof the transponder controller or processor, which may accomplishedthrough a firmware update in many cases.

In some embodiments, existing AIS transponders may continue to be usedwith little or no modification.

Although described herein with reference to e-Navigation and AIS, thepresent embodiments may also be applied to other known or futuremaritime communications signaling standards and protocols, such asVessel Traffic Service (VTS) and channels allocated specifically toapplication specific messaging (ASM). In such future embodiments, themessage wrapper may be significantly reduced or eliminated (e.g., forreasons of efficiency) leaving only necessary synchronization bits, somebasic identification of the message type in a binary field, in additionto the forward error protected payload. Furthermore, future messagetypes may also not require a stop flag, full message CRC, bitstuffing orguard bits.

The embodiments of the methods and systems described herein may beimplemented in hardware or software, or a combination of both. Theseembodiments may be implemented in computer programs executing onprogrammable computers, each computer including at least one processor,a data storage system (including volatile memory or non-volatile memoryor other data storage elements or a combination thereof), and at leastone communication interface.

Program code is applied to input data to perform the functions describedherein and to generate output information. The output information isapplied to one or more output devices, in known fashion.

Each program may be implemented in a high level procedural or objectoriented programming or scripting language, or both, to communicate witha computer system. However, alternatively the programs may beimplemented in assembly or machine language, if desired. The languagemay be a compiled or interpreted language. Each such computer programmay be stored on a storage media or a device (e.g., ROM, magnetic disk,optical disc), readable by a general or special purpose programmablecomputer, for configuring and operating the computer when the storagemedia or device is read by the computer to perform the proceduresdescribed herein.

Furthermore, the systems and methods of the described embodiments arecapable of being distributed in a computer program product including aphysical, non-transitory computer readable medium that bears computerusable instructions for one or more processors, where the instructions,when executed by the one or more processors, cause the computer toperform the functions described herein. The medium may be provided invarious forms, including one or more diskettes, compact disks, tapes,chips, magnetic and electronic storage media (e.g., firmware), and thelike. Non-transitory computer-readable media comprise allcomputer-readable media, with the exception being a transitory,propagating signal. The computer useable instructions may also be invarious forms, including compiled and non-compiled code.

It will be appreciated that numerous specific details are set forth inorder to provide a thorough understanding of the exemplary embodimentsdescribed herein. However, it will be understood by those of ordinaryskill in the art that the embodiments described herein may be practicedwithout these specific details. In other instances, well-known methods,procedures and components have not been described in detail so as not toobscure the embodiments described herein. Furthermore, this descriptionis not to be considered as limiting the scope of the embodimentsdescribed herein in any way, but rather as merely describingimplementation of the various embodiments described herein. The scope ofthe claims should not be limited by the preferred embodiments andexamples, but should be given the broadest interpretation consistentwith the description as a whole.

The present invention has been described here by way of example only,while numerous specific details are set forth herein in order to providea thorough understanding of the exemplary embodiments described herein.However, it will be understood by those of ordinary skill in the artthat these embodiments may, in some cases, be practiced without thesespecific details. In other instances, well-known methods, procedures andcomponents have not been described in detail so as not to obscure thedescription of the embodiments. Various modification and variations maybe made to these exemplary embodiments without departing from the spiritand scope of the invention, which is limited only by the appendedclaims.

We claim:
 1. A method of transmitting a message that identifies ageographical position, the method comprising: encoding a track historyto generate an encoded track history; generating the message comprisingthe encoded track history; and transmitting the message.
 2. The methodof claim 1, further comprising updating the track history based on thegeographical position.
 3. The method of claim 1, further comprisingupdating the track history based on course over ground.
 4. The method ofclaim 1, further comprising setting a message type identifier, whereinthe message further comprises the message type identifier.
 5. The methodof claim 1, further comprising retrieving a transmitter identifier,wherein the message further comprises the transmitter identifier.
 6. Themethod of claim 1, wherein the track history comprises one or more trackhistory points based on prior geographical positions.
 7. The method ofclaim 6, wherein each of the one or more track history points areseparated by a preselected time interval.
 8. The method of claim 6,wherein each of the track history points is determined with reference toa predetermined point layout that has a plurality of predefined pointpositions.
 9. The method of claim 8, wherein each of the track historypoints is selected by approximating the position associated with aprevious message to one of the predefined point layout positions. 10.The method of claim 8, further comprising selecting the predeterminedpoint layout from a plurality of possible point layouts.
 11. The methodof claim 8, wherein the predetermined point layout is selected bycomputing minimum mean square error of the one or more track historypoints relative to the predefined point positions in each of theplurality of possible point layouts.
 12. The method of claim 8, whereinthe predetermined point layout is selected by computing maximum error ofthe one or more track history points relative to the predefined pointpositions in each of the plurality of possible point layouts.
 13. Themethod of claim 8, further comprising selecting a first dimensionalcomponent value array, wherein the first dimensional component valuearray defines the values of successive points in the predetermined pointlayout.
 14. The method of claim 13, wherein the first dimensionalcomponent value arrays are defined in a first look up table having afirst index.
 15. The method of claim 14, wherein each track historypoint in the track history is identified at least by a respective indexvalue from the first index.
 16. The method of claim 15, wherein each ofthe respective index values is Huffman encoded in the track history. 17.The method of claim 13, further comprising selecting a seconddimensional component value array, wherein the second dimensionalcomponent value array defines the values of successive points in thepredetermined point layout.
 18. The method of claim 17, wherein thesecond dimensional component value arrays are defined in a second lookup table having a second index.
 19. The method of claim 18, wherein thefirst and second indices are selected in a point layout look-up table.20. The method of claim 6, wherein at least one of the track historypoints comprises an alert flag tag.
 21. The method of claim 1, whereinthe encoding comprises determining the track history is a partial trackhistory that includes less than a preset number of track history points.22. The method of claim 1, wherein the message comprises a binarypayload and wherein the encoded track history is comprised in the binarypayload.
 23. The method of claim 22, wherein the binary payloadcomprises at least one of an alert identifier, a course over groundindication, a latitude and longitude indication and a cyclic redundancycheck code.
 24. The method of claim 22, further comprising errorprotection encoding the binary payload.
 25. A method of processing amessage, the method comprising: receiving a first message, the firstmessage comprising a track history, wherein each of the track historypoints is determined with reference to a predetermined point layoutdefined in the first message, the predetermined point layout having aplurality of predefined point positions; and processing the firstmessage to obtain the track history.
 26. An apparatus for transmitting amessage that identifies a geographical position, the apparatuscomprising: a memory that stores a track history; a processor configuredto: encode the track history to generate an encoded track history;generate the message comprising the encoded track history; and atransmitter that transmits the message.
 27. A non-transitory computerreadable medium storing computer executable instructions, theinstructions, when executed by a computer processor, for causing thecomputer processor to perform a method of transmitting a message thatidentifies a geographical position, the method comprising: encoding atrack history to generate an encoded track history; generating themessage comprising the encoded track history; and controlling atransmitter to transmit the message.